The described exemplary embodiments generally relate to maintaining image registration in image processing. More particularly, the description relates to systems and methods in which image registration errors in output images are reduced in image processing systems that include tandem print engines. The tandem print engines, for example, can process single pass duplexing and/or multi-pass duplexing.
Electrophotography, a method of copying or printing documents, is performed by exposing a light image representation of a desired original image onto a substantially uniformly charged photoreceptor substrate, such as a photoreceptor belt. In response to this light image, the photoreceptor discharges to create an electrostatic latent image of the desired original image on the photoreceptor's surface. Developing material, or toner, is then deposited onto the latent image to form a developed image. The developed image is then transferred to an image receiving substrate. The surface of the photoreceptor is then cleaned to remove residual developing material and the surface as recharged by a charging device in preparation for the production of the next image.
Color images can be produced by repeating the above-described recording process once for each differently-colored toner that is used to make a composite color image. For example, in a one-color imaging process, referred to herein as the Recharge, Expose, and Develop, Image (REaD IOI) process, a charged photoreceptor surface is exposed to a light image that represents a first color. The resulting electrostatic latent image is then developed with a first colored toner. The toner is typically of a subtractive primary color, including magenta, yellow, cyan, or black. The charge, expose and develop process is repeated for a second colored toner, then for a third colored toner, and finally for a fourth colored toner. The four differently-colored toners are placed in superimposed registration on the photoreceptor so that a desired composite color image results. That composite color image is then transferred and fused onto an image receiving substrate.
Tandem print engine systems include two print engines arranged in a series configuration. Each print engine includes a photoreceptor belt and imagers disposed at spaced positions along the length, i.e., the process direction, of the photoreceptor belt. Each imager comprises an image source that exposes the photoreceptor belt. Typically, the image source includes a light emitting device that emits a light beam that is moved laterally across the photoreceptor belt to expose the photoreceptor belt to create a latent electrostatic image on the photoreceptor belt. Each latent image is then developed as outlined above. Image receiving substrates, such as sheets of copy paper, are fed in a time-controlled manner to the print engines. The first print engine transfers its developed image to the simplex side of the image receiving substrate. The image receiving substrate is then inverted and presented to the second print engine. The second print engine then transfers its developed image to the duplex side of the image receiving substrate.
Each photoreceptor belt of the first and second print engines includes a seam where opposed end portions of the photoreceptor belt are joined together. The photoreceptor belts include pitch regions in which images can be satisfactorily formed. Images cannot be satisfactorily formed at the seams, because the images formed at seams are normally defective. Accordingly, it is important to control the locations of the seams of both of the first and second photoreceptor belts during print runs, to prevent forming images at the seams, and to ensure that images are formed only in the pitch regions. A consistent and predictable placement of the photoreceptor belts, with respect to each other, is desirable in order to simplify an intermediate or inverter paper path between two print engines.
In a tandem print engine configuration, there are several technology issues involved with synchronizing two photoreceptor belt modules of two separate print engines in a manner that does not negatively impact the registration of either module. If the periods of revolution of the two photoreceptor belts are not matched, then the positions of the seams will also not be synchronized. The photoreceptor belts can have different lengths and, accordingly, in such configurations must rotate at different velocities (speeds) to maintain the same periods of revolution. If the periods of revolution are not synchronized appropriately to each other or with imager velocities, image to paper registration errors will occur during printing. The image to paper registration errors can be characterized as 1) simplex to duplex image registration errors if the photoreceptor and imager velocities for each print engine are not matched appropriately, or 2) image-on-image (IOI) registration errors from changes in the photoreceptor velocity or imager velocity while printing is occurring. Image-on-image registration errors occur during the building of color images on the photoreceptor belts. If, during stacking the multiple color separation layers of a color image on each other, the images are not aligned with each other, then image registration errors between the color separation layers will occur. These registration errors produce print defects such as color shifts and trapping errors.
Registration errors are caused generally by the motion quality of the photoreceptor belts and the manner that the imagers form the latent images on the photoreceptor belts. Regarding the motion quality of the photoreceptor belts, image registration errors can be caused by changes in the photoreceptor belt velocity, making it difficult to form images smoothly and to align lead edges of the images on the photoreceptor belt. Velocity changes can occur due to various different factors, including errors of the drive motor, errors in roller velocities and diameters, belt length changes during operation due to tension and thermal effects, and normal roller and belt tolerances.
Factors that can cause registration errors in the manner in which the imagers form the latent images, include errors in the lateral scan velocity, i.e., the exposure velocity, of the image sources across the photoreceptor belt, the scanning start and end points of the scanning light beam, and the length of the scan lines.
In simplex (single print engine) configurations, the image registration can be set up off-line. Thus, adjustments can be made at times when print runs are not being performed. In such configurations, the photoreceptor belt velocity is maintained as constant as possible to minimize registration errors. In addition, the imagers are set to a specific reference and their velocity is tightly maintained. If, during the course of producing an image, the velocity of the photoreceptor belt and the scan velocity of the image sources of the imager vary with respect to each other, either in position or velocity, then registration errors will occur.
Simplex print engine systems can include monitoring systems for measuring and compensating for image registration errors. Simplex print engine systems can calibrate themselves to the characteristics of the photoreceptor belt to achieve good image alignment for color images. If the photoreceptor belt runs either too fast or too slow, the scan velocity of the image sources can be automatically adjusted to counter the change in the photoreceptor belt velocity. As long as the photoreceptor belt velocity is maintained substantially constant, then only small image registration errors occur due to the self-correcting measures that are taken by the system.
For tandem print engine configurations, however, the synchronization requirements for the two print engines require that the photoreceptor belt velocity of the downstream print engine, i.e., the “slave print engine,” must be adjusted to keep it timed with the period of revolution of the photoreceptor belt of the upstream print engine, i.e., the “master print engine,” Otherwise, it is not possible to control the locations of the seams of the photoreceptor belts of the master and slave print engines. As explained, it is important to control the seams to prevent the formation of images on the seams.
In tandem print engine configurations, various factors can cause the two photoreceptor belts to be out of synchronization with each other. Namely, the photoreceptor belt velocities and lengths can change over time due to changes in the roller diameters, encoder diameters and thermal effects. The belt length can be out of specification originally and can also vary during operation due to stretch caused by tension and thermal effects. The encoder roller that measures the belt velocity can change in diameter due to thermal effects. Consequently, the photoreceptor belts can run at different periods of revolution. In addition, errors can occur between the scan velocities of the image sources of the imagers of the different print engines. However, as outlined above, the scan velocities of the imagers also need to be coordinated with the velocity of the associated photoreceptor belt to maintain proper overall image quality.
In order to synchronize the photoreceptor belts of the master and slave print engines, the photoreceptor belt velocity of the slave print engine can be changed. In making such adjustments for the slave print engine, the slave print engine should be adjusted on-line. Otherwise, the productivity of the tandem print engine is decreased.
One possible approach to making such velocity adjustments while the slave print engine is on-line includes making the velocity adjustments for the slave print engine sufficiently small that the adjustments would produce registration errors so small that they would be almost imperceptible. This approach, however, requires stringent adjustment resolution or quantization levels in the photoreceptor belt and in imager controllers of the slave print engine, because both subsystems will need to be adjusted when the photoreceptor belt velocity is adjusted. The cost implications of such fine adjustment capability are high.
A high level of resolution is presently achievable for the slave print engine photoreceptor belt module. Velocity resolutions down to about 1/64 Hz (or 0.00082%) can currently be achieved. Such small changes are expected to be imperceptible. Thus, the photoreceptor belt velocity of the slave print engine could be adjusted slowly at a sufficiently small step size without undue registration errors occurring.
It is not, however, presently possible to satisfactorily reduce the image registration errors by making such small step size adjustments of the photoreceptor belt velocity for the slave print engine. That is, in tandem print engines, the ratio of the velocity of the photoreceptor belt and the velocity of the imagers, for example the scan velocity, or exposure velocity, of image sources, defines the absolute magnification of the final image that is formed on the photoreceptor belt. Accordingly, if the photoreceptor belt velocity is changed, then the imager velocity must also be changed to maintain the desired ratio, or else the length of the image in the process, or slow scan, direction will change. Consequently, the imager velocity must be adjusted to maintain the desired absolute magnification, to maintain the ratio of the photoreceptor belt velocity to the imager velocity.
Imager controllers can have, for example, 32, 64, 128 or 256 discrete levels of imager scan velocity adjustment for the light emitting devices. With 256 steps over the adjustment range that is desirable for imagers, which is typically about 1.6%, the adjustment resolution is about 0.0125% per step. This adjustment resolution is very coarse, and is about fifteen times greater, compared to present adjustment capabilities of photoreceptor belt controllers. This adjustment resolution would cause significant image registration errors if changes were made to the imager velocity during a print run. However, improving upon this adjustment resolution of the imagers is not a satisfactory solution to this problem, because, as the number of adjustment level increases, the more difficult the adjustment implementation becomes and the more expensive the adjustment system generally becomes.
Adjusting the velocities of the imagers at the coarse adjustment capabilities of the imager controller is also unsatisfactory. That is, in order to avoid large registration errors, it would be necessary to make changes to the imager velocity only at times when print runs are not being performed, i.e., when the slave print engine is off-line. This approach would require that the slave print engine be taken off-line periodically and skipping one revolution of the photoreceptor belt to adjust the imager velocity. This approach would create a decrease in the tandem print engine productivity, as the master print engine would also have to go off-line at the same time. In addition, this approach would also add additional complexity to the machine communications and scheduling algorithm needed for tandem print engine configurations. Accordingly, making adjustments to the imager velocity off-line would also be unsatisfactory.
One possible approach to making such velocity adjustments while the slave print engine is on-line includes matching the periods of revolution of the photoreceptors of the master and slave print engines during print runs, by simultaneously adjusting both the velocity of the slave photoreceptor and imagers of the slave engine. The velocity controllers for the slave photoreceptor and imagers can have the same dynamic response and can be simultaneously actuated, to minimize incremental registration errors in the slave print engine. Cross reference is made to commonly assigned U.S. Pat. No. 6,219,516, the disclosure of which being totally incorporated herein by reference.
As discussed in greater detail below, changes in the ratio between the velocities of the photoreceptor belt and the imagers in a print engine cause image to paper registration errors in the print engine. A phase difference between the master print engine and the slave print engine due to an intermediate inverter also causes registration errors. The phase difference represents a transit time for the substrate to travel through the inverter.
The velocity adjustments can thus be made at an adjustment level that can be achieved by the controllers of both the photoreceptor and the imagers. Thus, even in systems in which the adjustment resolution capabilities of the two subsystems vary significantly, the adjustments to both systems can be made at an adjustment level that is achievable by both systems.
Because it is not necessary to take the slave print engine off-line periodically to make such adjustments, the systems and methods hereinafter described can improve productivity in tandem print engine configurations. The systems and methods described avoid the need to introduce additionally complex machine communications and scheduling techniques that would be needed to be able to make adjustments off-line in tandem print engine configurations. The exemplary embodiments also avoid the need for an intermediate buffer tray to hold substrates while they move from the master print engine to the slave print engine.